Three Roads To Quantum Gravity

CHAPTER 13
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HOW TO WEAVE A STRING
Perhaps the main reason why some physicists do not get very
excited about loop quantum gravity is that, although it
succeeds very well in describing how the geometry of space
must look on the Planck scale, it is basically pretty boring.
There are no new principles involved. To set up the theory we
just put in the basic principles of quantum theory and
relativity. We get a lot out that is new and could even be
tested experimentally. But it is perhaps not so surprising that
when geometry is treated quantum theoretically it behaves
like a quantum theoretic system. Things that used to be
continuous, such as the range of possible volumes a space
could have, now become discrete. The main lesson is that we
really can treat space and time in a background independent
way, and see them as a nothing but a network of relationships.
This is good, but this is also what the principles we put in
demanded. That it works is a good consistency check, but we
should not consider it either surprising or revolutionary. The
main strength of this approach, its simplicity and transparency,
is perhaps also its main weakness.
String theory is just the opposite. We start not with basic
principles, but by contradicting the thing we feel most certain
about quantum gravity ± that it must be a background
independent theory. We ignore this, and search for a theory
of gravitons and other particles moving against a background
of empty space; and, by trial and error, we ®nd it. Our guiding
principle is to ®nd something that works. To do this we have